1. Field of the Invention
The present invention relates to aromatic diamine compounds. More particularly, the present invention relates to an aromatic diamine compound which can be used as a hole transport material for organic EL (electroluminescence) device, a hole transport material for an organic thin film photocell, a carrier transport material for electrophotography, etc.; as well as to an aromatic diamine compound which is useful as a precursor for the synthesis of the above aromatic diamine compound.
2. Prior Art
As the aromatic diamine compounds used for the above applications, there can be cited, for example, 1,1-bis(4-di-p-tolylaminophenyl)cyclohexane represented by the following formula (3), used as a hole transport material for organic EL device [U.S. Pat. No. 4,539,507] ##STR3## and N,N'-diphenyl-N,N'-bis(m-tolyl)benzidine represented by the following formula (4), used as a hole transport material for organic EL device [Appl. Phys. Lett., 51(12), 913 (1987)]. ##STR4##
The compounds represented by the above formulas (3) and (4), however, have low melting points [174.degree. C. in the case of the compound (3) and 165.degree. C. in the case of the compound (4)]and low glass transition temperatures [77.7.degree. C. in the case of the compound (3) and 66.5.degree. C. in the case of the compound (4)]; as a result, when they are used in an organic EL device and the EL device is operated, they cause crystallization by the heat generated during the operation and the hole transport layer is deformed and an emission reduced. Thus, it has been required to develop a new material which has a high melting point and a high glass transition temperature and which is unlikely to cause crystallization.